Accurate Heats of Formation for Polycyclic Aromatic Hydrocarbons: A High-Level Ab Initio Perspective

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are key reference materials for the validation and parameterization of computationally cost-effective procedures such as density functional theory (DFT), semiempirical molecular orbital theory, and molecular mechanics. We obtain accurate heats of formation (ΔH f,298) for 20 PAHs with up to 18 carbon atoms by means of the explicitly correlated W1-F12 thermochemical procedure. The heats of formation are obtained via atomization reactions and quasi-isodesmic reactions involving CH4, C2H4, and C6H6 for which accurate experimental ΔH f,298 values are available from the active thermochemical tables (ATcT) network. We show that for large PAHs, the differences between W1-F12 heats of formation obtained from atomization and quasi-isodesmic reactions increase with the size of the system and range between 1.7 (C7H8) and 8.9 (chrysene, C18H12) kJ mol–1. This suggests that atomization reactions should be used with caution for obtaining heats of formation for medium-sized systems even when highly accurate thermochemical procedures (such as W1-F12 theory) are used. For eight PAH compounds (toluene, indene, acenaphthylene, biphenyl, diphenylmethane, anthracene, pyrene, and chrysene), our best theoretical values agree with the best experimental values to within ∼1 kJ mol–1; for six additional systems (indane, naphthalene, biphenylene, acenaphthene, phenanthrene, and m-terphenyl), the agreement between theory and experiment is good with deviations ranging between 2 and 4 kJ mol–1. However, for four systems (p-terphenyl, fluorene, pyracene, and pyracyclene), our best W1-F12 values suggest that the experimental ΔH f,298 values should be revised by significant amounts ranging from 6.5 and 24.4 kJ mol–1.

Benchmark Thermochemistry of Polycyclic Aromatic Hydrocarbons

“…DLPNO-CCSD­(T)/CBS , ref : hydrogenation reactions (4) and (5). W1-F12 , ref and W1h , refs and : reactions (1) and (2). W1X-1 , ref : reaction (3).…”

“…A comparison of the results of the above calculations − shows that in most cases, the discrepancy between the estimated enthalpies of formation is small and does not exceed 1–3 kJ/mol. The larger deviations are observed with the results of W1-F12 calculations obtained from atomization reactions.…”

“…More attention should be paid to the discrepancies exceeding 4 kJ/mol obtained for the calculations of approximately the same level. The difference values between the calculated enthalpies of formation of chrysene, 12,16 triphenylene, 10,16 benzo[c]phenanthrene, 14,16 and corannulene 10,16 are 4.6, 5.0, 4.6, and 6.1 kJ/mol, respectively, which does not allow us to recommend benchmark values for these compounds.…”

“…It should be noted that these uncertainties are close to those obtained following the recommendation to provide the estimates of 95% confidence intervals for the calculated results and assuming the uncertainty of the method to increase from 1 to 3 kJ/mol with the increase in molecule size. The uncertainty of 1 kJ/mol for the smallest molecules was estimated from comparison with the results obtained for benzene and cyclopentadiene (Table S3); for larger molecules, the performance of the method was estimated from a comparison with reported calculations for PAHs − and the discrepancies with experimental data.…”

“…The quantum chemical models that allow highly accurate estimation of enthalpies of formation of PAHs containing more than 15 heavy atoms have been developed only in the last decade leading to publications on accurate thermochemistry of these compounds. Using high-level ab initio W1h and W1-F12 thermochemical protocol, Karton et al − predicted the enthalpies of formation of about 20 fused PAHs and estimated the enthalpy of formation of C 60 fullerene using the Δ f H 298 ° values calculated for corannulene, sumanene, and other PAHs. To estimate the enthalpies of formation of PAHs, two types of reactions were considered normalP normalA normalH + a normalC 2 normalH 4 → b normalC 6 normalH 6 for PAHs involving only sp 2 carbons and normalP normalA normalH + a normalC normalH 4 → b normalC 6 normalH 6 for PAHs involving both sp 2 and sp 3 carbons.…”